As is well known, the aircraft engine industry has witnessed significant improvements in thrust specific fuel consumptions (TSFC) by incorporating active clearance controls on the engines. As for example, the JT9D engine manufactured by Pratt & Whitney Aircraft of United Technologies Corporation, the assignee of this patent application, has been modified to include the active clearance control described and claimed in the Redinger et al U.S. Pat. No. 4,069,662 also assigned to this assignee. In that embodiment spray bars are wrapped around the engine case at judicious locations and fan air is actuated to impinge on the engine case so as to cool and hence shrink the case and move the outer air seals, which are attached thereto, toward the tips of the turbine blade. As is referred to in the industry, this is an active clearance control system since the impinging air is only on during certain modes of the engine operating envelope. This is in contrast to the passive type of system that continuously flows air for cooling certain engine parts.
With the utilization of the active clearance control at given locations in the engine, the performance of the engine has increased by more than two (2) percentage points in terms of TSFC. Obviously, it is desirable to minimize the gap of all the rotating blades, since any air escaping around the blades is a penalty to the overall performance of the engine.
This invention is directed to an active clearance control for the compressor blades and operates internally of the engine, rather than externally. Also, this invention contemplates heating the bore of the compressor so as to cause the blades to expand toward the peripheral seals so as to minimize the gap therebetween. Compressor bleed air which is at a higher pressure and temperature than the incoming air is conducted into the bore of the compressor in proximity to the engine's centerline where it scrubs the compressor discs and flows rearwardly to commingle with the working fluid medium. This air may also be utilized for other cooling purposes on its travel toward the exit end of the engine. As for example, this air may be utilized for cooling or buffering the bearing compartment.
This invention contemplates judiciously bleeding the 9th and 13th stage of the multistages of the compressor and leading this air forward of the compressor where it is introduced at the most forward end of the compressor adjacent the engine centerline. The cooler air from the 9th stage is introduced at takeoff and the warmer air from the 13th stage is introduced at cruise. Inasmuch as the warmer air causes thermal growth of the compressor discs, the blade tip gaps are reduced with a consequential improvement in engine performance.